In previous versions of Ubuntu, picking one of the default wallpapers meant that you saw it on the login screen as well.
I am now on a clean install of Ubuntu 14.04, the LightDM wallpaper appears to ...

I have no /etc/lightdm/unity-greeter.conf file. but there is /etc/lightdm/lightdm.conf and /etc/lightdm/users.conf but there isn't a background=path/to/image line in either of those files.
Related:
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Ubuntu 12.04 Unity greeter had introduced the selected user dynamic wallpaper which allowed, when selecting a user from the available users list in the login screen, the wallpaper to change to reflect ...